1. Field of the Invention
The present invention relates to ultrasonic profiling of weld samples.
2. Background Art
Resistance spot welds are created by passing current between two welding electrodes on opposite sides of metal sheets being joined. The resistance to electrical current flow at the interface between the sheets causes localized heating that melts the base metal creating a spot weld at the joint. At present, common methods for inspecting spot welds in automotive manufacturing are pry checks and physical tear-downs, during which spot-welded joints are pried apart. The resulting weld buttons are visually inspected or measured with calipers. Although these methods have been used successfully for decades, destructive weld testing has some drawbacks including costs associated with scrap material and the time lag between the onset and identification of problems. In addition, pry tests and tear-downs do not allow personnel to easily collect inspection data that would allow them to identify trends and potential problems. Recent industry studies have demonstrated cost savings associated with implementing non-destructive evaluation (NDE) which derive from finding defects early in the production process and reducing waste compared to destructive testing.
Another problem with destructive inspection techniques is that they are not viable options for lightweight and high-strength materials. Composite structures with adhesive-bonded joints cannot be pry checked, and aluminum is relatively expensive and more difficult to rework than steel making pry checks and tear-downs cost prohibitive. Welds in high-strength steel are often too strong to be pry checked or torn down, and satisfactory welds sometimes fail inter-facially rather than by pulling a weld button, making it difficult for inspectors to distinguish between satisfactory and discrepant welds. Development of NDE techniques is therefore an enabling technology for greater use of lightweight materials in the automotive industry where NDE methods to assure product quality are essential for industry and consumer acceptance of new materials and manufacturing methods.
Ultrasonic weld profiling is a non-destructive testing technique that allows welds to be sized and discrepant welds to be identified. This technique involves measuring the ultrasonic energy transmitted and reflected at the welded interface. Traditional ultrasonic spot-weld inspection systems use conventional high-frequency single-crystal probes working in pulse-echo mode. The output from these mono-probes is a single signal that is an integrated response over an area that depends on the diameter of the probe. Different probes are used for different sized welds. In contrast, a phased array includes many piezoelectric elements that are individually excited by electronic pulses at programmed delay times. As a result, phased arrays have several advantages over conventional ultrasonic probes that derive from the ability to dynamically control the acoustic beam transmitted into the structure under examination. An electronic delay can be applied separately to each electronic channel when emitting and receiving the signal. These delay laws permit constructive and destructive interference of the acoustic wave-front transmitted into the structure, allowing predefined ultrasonic beams to be formed. The acoustic energy can be focused, and delay laws can be used to steer the acoustic beam. Electronic scanning is accomplished by firing successive groups of elements in the array. Electronic scanning combined with mechanical translation of the probe allows several thousands of signals to be measured and analyzed to produce high-resolution two-dimensional images of the welds.
For automotive manufacturing applications, inspection systems have to be robust, light, easy to use, and small enough to permit access to welds in tight corners. A problem with using a motorized mechanical system to move the probe is that it has to be a miniature system meeting size and weight requirements, which increases the initial cost of the probe assembly as well as maintenance and repair costs. A motorized mechanical system is also unattractive because of the high potential for damage or mechanical problems in harsh manufacturing environments. For example, speed irregularities of the motor used for probe translation will cause the weld images generated by the probe to not be accurate as irregularities in motion or variance in speed of the probe is of consequence.
Further, an ultrasonic probe has to be maintained in water as ultrasonic waves do not propagate properly in air. As such, a water path between the probe and the part under inspection has to be maintained. A complication with using a motorized scanning system is that the motor has to be isolated from the water thereby greatly complicating the design of the probe housing. Therefore, a method and system for translation of an ultrasonic probe across a part under inspection which do not require motorized moving parts is desirable.